Integrated optical devices are used, as known, in communications networks that use light signals for the transmission of information. The most common optical devices are modulators, attenuators and interferometers.
FIG. 1 of the attached drawings shows an integrated optical device 1 in an initial manufacturing step. In order to form the represented structure, a first silicon dioxide (SiO2) layer 3 is formed on a silicon support substrate 2, usually by deposition, said coating layer or lower cladding, doped with impurities of a type and concentration such as to obtain a predetermined refraction index ncladding. A layer 4 is deposited on the layer 3, said core layer, again made from silicon dioxide, but doped with a type and concentration of impurities different to those of the underlying layer so that the refraction index of the core is greater than the refraction index of the cladding (ncore>ncladding). The core layer is subjected to a known photolithographic process, after which the plot of the cores of the wave guides of the designed optical device is obtained. The last step necessary for the formation of the optical circuit is the deposition of a coating layer or upper cladding 5. Of course, this layer covers the top and side of each track of the plot formed by the core layer. The upper layer 5 is preferably made from silicon dioxide doped with the same type and the same concentration of impurities with which the lower coating layer is doped, so as to have the same refraction index ncladding. Under the condition ncore>ncladding the light propagates substantially inside the optical paths defined by the cores.
In a standard manufacturing process of an integrated optical device the subsequent step is the formation of further structures 6 of the integrated optical device. An example of these structures is shown in FIG. 2, which also shows a masking layer 7 with an opening for the etching of a trench.
The last step of the process is the formation of trenches in the multilayer consisting of the layers of oxide 3, 4 and 5.
A trench can have different functions: for example, it can make a physical insulation, both from the optical point of view and from the thermal point of view, between two optical paths that are adjacent or in any case very close to each other, or else it can thermally insulate one or more optical paths from heat sources present on the device itself or it can be used to make an optical switch. In this case the trench is filled with a medium the characteristics of which, in terms of refraction index and propagation constant, can be modified so as to be selectively equal to or less than those of the cores of the wave guides of the interrupted optical path. In this way it is possible to close or open the path of the light signal.
The trench that one wishes to make typically must cut the entire thickness of the multilayer of oxide that, as stated, comprises three layers: the lower and upper coating layers 3 and 5, both typically having a thickness of about 16 μm, and the core layer 4, on the other hand, having a thickness that can be between 4 μm and 8 μm. In total, therefore, the depth that typically must be reached with the etch is about 36-40 μm. Typically, for these depth values (a few tens of μm) a trench is considered deep; on the other hand, for depths of a few μm a trench is considered not very deep, i.e., shallow.
For the etching of a trench, currently techniques are used that foresee the deposition on the multilayer of a layer of a material resistant to the attack of the oxide, the definition on it of a mask 7, like the one shown in FIG. 2, which leaves uncovered the area 8 of the multilayer corresponding to the mouth of the trench, and an anisotropic attack for the removal of the oxide through the non-masked areas. This step is typically carried out with a dry attack through plasma (plasma dry etching). Plasma techniques allow a highly anisotropic attack to be carried out, i.e. having an attack speed in the vertical direction, or more precisely perpendicular to the surface to be attacked, much greater than the attack speed in the lateral direction. This allows high-resolution images to be transferred even when the layer of material to be removed has a high thickness. Currently, for the attack through plasma both old-style machines that carry out low-density plasma attacks, and modern-style machines that carry out high-density plasma attacks, are used. The difference between the two types of attack lies in the amount of species contained in the plasma that react with the material to be removed. The low-density plasma attack is substantially of the chemical type: a chemical reaction between the chemical reactants contained in the plasma and the material of the multilayer causes the removal of material from the multilayer itself. The high-density plasma attack can, on the other hand, be considered both chemical and physical: combined with a chemical reaction there is an actual abrasion of the oxide due to particles present in the plasma that bombard the surface to be removed. In the same time period a high-density plasma attack reaches a greater depth than a low-density plasma attack. Old-style machines are more common and easier to find on the market, and therefore they have a relatively low cost. Modern-style machines, being technologically more advanced, have a greater cost and are put onto the market by few suppliers.
As stated previously, the attack step of the multilayer through plasma is preceded by a process for the definition of a mask that leaves unprotected the areas in which the trenches are to be etched. The mask typically must keep its profile as unaltered as possible under the action of the plasma and typically must have high selectivity.
If the material constituting the masking layer is such that the profile of the mask during the attack step remains unaltered, the attack shall be substantially vertical and shall allow the desired resolution to be obtained. Here, resolution means the measurement of the fidelity of transfer of the image from the mask to the attacked material. The trenches shall thus have substantially vertical walls, i.e. with inclination of at least 88° with respect to the surface plane of the silicon substrate.
The selectivity is the ratio between the attack speeds on different materials: in this case it is the ratio between the attack speed of the oxide of the multilayer and the attack speed of the masking material. In common processes this ratio can be between 5:1 and 8:1. This means that the attack of the oxide is from 5 to 8 times quicker than the attack of the masking material. Ratios of this magnitude are typical of masks of low selectivity. For greater ratios (even of 10:1, 20:1) a mask is considered to have good selectivity; for ratios of 100:1 a mask can be considered to have infinite selectivity.
In standard processes the masks used are of photosensitive organic polymeric material, commonly known as photoresist. This type of material is not very resistant to the action of the attack through plasma, be it high or low density so that it reacts chemically with the plasma. In particular, during the attack step it deteriorates considerably at the edges that define the unprotected areas to be etched. For example, if in the structure of FIG. 2 the masking layer 7 is considered as a photoresist layer, this phenomenon of erosion of the edges, or “tip” effect, progressively modifies the profile of the mask to such a point that in the oxide a trench with vertical walls is not obtained but rather, as represented in FIG. 3, an incision 9 with oblique or in any case irregular walls. Typically, this drawback is limited by increasing the thickness of the masking layer. Unfortunately, current materials and equipment typically do not allow photoresist thicknesses of any more than 6-7 μm to be obtained. Moreover, during the definition of a mask in a layer of photoresist of such a thickness, it is often difficult to obtain sharp and regular profiles of the edges, but usually the profile extends beyond the desired limits. The result is a reduction in the area of the multilayer exposed to the attack, for which reason the design specifications relative to the size of the trenches may not be respected.
As an alternative to organic masks, so-called hard masks can be used, for example masks made from metallic material. A metal mask 10, like the one represented in FIG. 4, ensures substantially infinite selectivity (attack speed ratio 100:1) and greater protection against the action of the plasma since the chemical composition of the metal does not react with the chemical attack agents. Therefore, the metal mask 10 keeps its profile substantially unaltered during the attack.
In the case in which the area of the multilayer not protected by the mask of metallic material is a very small percentage of the area of the device (roughly less than 25%), a large amount of metallic material is exposed to the attack. It follows from this that numerous metallic particles are gradually removed from the mask and find themselves free in the plasma itself, for which reason they slow down the action of the plasma. This effect, known as “resputtering”, often does not allow the attack to be completed up to the desired depth. Moreover, a large amount of metal exposed directly to the plasma may become electrically charged during the attack, and this can cause further drawbacks from the electrical point of view. The trenches obtained, like the trench 11 shown in FIG. 4, consequently have imperfections on the side walls, especially in the deepest zone in which the attack may not be completed.